Hi, I can't answer all of your questions but I'l answer what I can!
In all honesty, not many people know what is happening with John Snow and Stephenson moving over to the city campus. I think John Snow are joing with JBo (Butler), or maybe sharing their accomodation space. I think your best option would be to apply to JoBo or Cuths.
JoBo is all ensuite and all selfcatered, and whilst it is a hill college its verrry far up the hill, especially from town.
Although I may be slightly biased in my answer, having lived at the Parsons site and studying at both the Science site and Elvet riverside,I have to say Parsons is in the perfect location. It's 10-15 mins from the science site, 5 mins from elvet, and 5-10 mins from town. It looks far out on a map but it honestly isn't, its on the same side of town as Hild Bede but not as far out. Most rooms are ensuite, and even though some are shared it really doesn't make a difference. I actually found that the shared accomodation made for easier socialising between flats and between flatmates, as brooks house (the ensuite block) could be quite isolating at times bc you don't really have to leave your room. At Parsons you have the option to go self or part catered and can buy meal tickets/packages.
If you don't get your first choice college yo'll be reallocated to one that most matches your original choice, but unfortunatly you can't request to change unless you have proper reasons, eg medical issues. I don't know how colleges are allocated because the current fresher intake for cuths has loadsss of maths and music students so it oculd be random or subject specific.
Sorry for such a long answer, if you have any other questions about life at Durham or any Cuths specific questions feel free to pm me